Dual chamber pacemakers sense electrical P-waves which occur in the atrium of the heart and pace the ventricle in synchronism with these signals in the absence of corresponding natural ventricular R-wave signals. In operation, an electrode disposed in the atrium detects a P-wave and, in the absence of corresponding ventricular activity, applies a delayed, synchronized electrical signal to cause the ventricle to contract. The dual chamber pacemaker thus "tracks" atrial signals by applying corresponding pacing signals to the ventricle.
Atrial P-waves having a relatively high rate will cause a dual chamber pacemaker to stimulate the ventricle at a corresponding high rate. As a matter of safety, it has been suggested that such pacemakers should pace the ventricle in response to high rate atrial events up to a defined ventricular rate limit, in order to avoid overstimulating the ventricle.
In order to avoid prolonged pacing at the ventricular rate limit, it has been suggested that the rate of pacing of the ventricle may be gradually reduced from the ventricular rate limit in a programmed manner to a predetermined lower pacing rate. Alternatively, the average pacing rate of the ventricle could be reduced in accordance with known operation of the pacemaker in a Wenckebach manner.
The known methods of pacing the ventricle in response to high rate atrial signals require that the pacemaker track the atrial signals and pace the ventricle at an increased rate. This operation is particularly undesirable if the high rate atrial signals which are being tracked are generated by a non-physiological source, for example a microwave transmitter. Even if the high rate atrial activity is physiological in origin, as in an atrial tachycardia, it may not be desirable to pace the ventricle of a diseased heart at a corresponding high rate.
It is therefore desirable to provide a pacemaker which will detect and ignore atrial activity above a predefined rate and which will pace the ventricle at a desirable lower rate and will inhibit pacing of the atrium in the presence of such atrial activity.
It is another object of the invention to provide a pacemaker which will track atrial signals occurring at a rate less than a selected atrial rate limit and which will pace the heart in a demand fashion at a desirable lower ventricular pacing rate if atrial signals occur at or above the atrial rate limit.
It is a further object of the invention to provide such a pacemaker with an atrial timer for detecting high rate atrial signals and a microprocessor control for pacing the ventricle at a predetermined preferred rate in response to the detection of the high rate atrial signals.
It is another object of the invention to provide a software-controlled pacemaker which detects high rate atrial events and prevents pacing of the atrium when such events are detected.
Conventional pacemakers use an atrial refractory interval following a paced or sensed ventricular event to avoid tracking spurious signals conducted from the ventricle to the atrium. Such atrial refractory intervals are usually "absolute" in the sense that atrial signals cannot be detected over the entire interval. The absolute refractory interval has the advantage of preventing the detection of spurious signals conducted from the ventricle, but it can also mask real atrial signals and thus unnecessarily inhibit tracking of these signals. Reducing the duration of the absolute atrial refractory interval has the desirable effect of increasing the chance of detecting real high rate atrial signals and the undesirable effect of increasing the risk of detecting spurious signals from the ventricle.
It is therefore desirable to reduce the duration of the absolute atrial refractory interval of a dual chamber pacemaker without also increasing the risk of tracking spurious signals from the ventricle.
Accordingly, it is an object of the invention to provide a pacemaker which facilitates tracking of high rate atrial events by using an atrial refractory interval following a paced or sensed ventricular event which includes an absolute refractory subinterval during which atrial events are not detected and a relative refractory subinterval during which atrial events are detected but are not tracked.